US1990589A - Alloy steel - Google Patents

Description

I 40 nitrogen content be at least thi v l I nitrogen as indicated by these tests is most con- 40 Patented Feb. 1 2 1935 i I v UNITED STATES, PATENT OFFICE.

- ALLOY STEEL Russell Franks, Jackson Heights, N. Y., asslgnor to Electro Metallurgical Company, a corporatlon of West Virginia No Drawing. Application July 29,1931,

- Serial No. 553,889

'1 Claims. (Cl. 15-1) My invention relates to chromium steels, reor higher chromium content. The presence of ferring more specifically to a means of improving other alloyed constituents in the steel, such as such steels, nickel, in no way limits or decreases the benefits It is the object of my invention to improve to be derived from the use of nitrogen. This 5 the physical properties of alloy steels containing invention is therefore applicable to high ehroe 5 a high percentage of chromium, and I attain 'miumsteels generally, is object by applying my discovery that the Experiments which I have made demonstrate addition of nitrogen in alloying quantities apprethat the addition o nitrogen in amounts pp ciably greater than those normally present in ciably greater than those normally present in steels affects the physical properties of highsteels refines the grain-structure and improves l0 chromium steels favorably and to a marked dethe streng o h c m u st e s without gree. materially affecting their ductility, machinabil- It is a matter of common observation and exty, a s corrosion resistance, or forging. perience that cast, high-chromium teels e nrolling and working characteristics. For insist of oarse crystals and grains loosely bound stance, my tests show that the addition of about 15 together. Castings of this metal are weak and to about 0.4% o alloyed nitrogen to actually porous, as shown by hydraulic tests at wrought and cast c r i m ls c n aini high pressure. This coarse structure in cast r m abo t 8% t ab ut 3 Ch omium and high-chromium steel is to a considerable extent 111) to about 2% carbon g y improves the responsible for the comparatively poor physical p y properties of these s e ls- The bene- 0 properties of the metal. A r fine ent of the ficial eiiect of the added nitrogen is most marked grainistructure is greatly to be desired for the in those steels containing less than about 1% production of steels with superior physical propf ar but n inu to e' ppr a le when erties. the carbon is raised as high asabout 2%. i

to l have found that the presence of nitrogen In on s ri s f xper men s a number of in amounts greater than about 0.2% materially Wrought steel test amples were made, each improves the physical properties of wrought and sample comprising a preponderance of ir n. cast steels which contain a large percentage of I about 0.1% of carbon, a u manga chromium. Although wrought chromium steels about 0.5% sil n a w percentage f imp do not, have the coarse grain-structure of the ties, and added Chromium and nitrogene 30 cast metal, the application of my invention pronitrogen was introduced into the molten steels duces marked improvements over the normally in the form of Chromium nitride and a h good structure of these steels. My invention is t o ferro-ohlome' y- For e c hi therefore applicable not only to cast, but also to nit samp s a there wa p pa d a wrought chromium steels, and produces a suo r spo d ng 10W nitrogen. steel having eDP 35 perior product in either case. mately the same composition in respect to every According to my invention, some benefit may constituent except nitrogen. All of the samples be had by the use of a nitrogen content somewere then Subje ed t a. number of p y a t sts what less than 0.2%, but it is preferable that the under C p b Conditions- T e ef ect of general, it is desirable to have the maximum veniently shown by means Of the fOHOWiIlg table: amount of nitrogen present that can be retained in stable combination at the temperatures which Analysis Physical properties are required for casting the metal. The amount of nitrogen which can be retained in the final %Cr %N Y 1 'r. s. El.% R.A.% Eric. Bxliiell 45 product increases with increased chromium content; thus, a steel containing about 15% chro- 19 0.04 46 77 30 7 5 m mium will retain up to approximately 0.2% 19 0.28 as 2o at 6:5 nitrogen, while a steel containing about 35% 23 M3 47 68 31 52 7 5 128 50 chromium .will retain up to approximately 0.65% 28 0.87 51 89. 26 67 810 149 50 nitrogen 2o 0. cs 50 71 21 a9 7. 5 128 My invention is especially valuable when ap- 26 83 24 45 128 plied to steels containing from about 15% to 35% of chromium, although it is of beneficial In the above table of wrought steels, the foleflect when applied to steels of somewhat lower lowing symbols are used: Y. P. for yield point in thousands of pounds per square inch, T. S. for tensile strength in thousands of pounds per square inch. El. for percentage elongation obtainable in 2 inches, R. A. for percentage reduction in area of cross section accompanying the elongation, Eric. for the values obtained using the Erichsen machine, and Brinell for the hardness values on the Brinell scale.

Amicroscopic examination of the wrought steels showed that nitrogen greatly refines the structure f the metal and appears to eliminate the grain boundaries of the crystals. Further testing of the steels demonstrated that the addition of nitrogen decreases their tendency to become embrittled at elevated temperatures, such as those of the order of 475 C.

A corresponding series of tests were made on cast high chromium steels. A series of castings containing a preponderance of iron, a low percentage of impurities, about 15% to 35% chromium, about 0.5% of carbon, and with high and low nitrogen contents, were cast in the form of stand ard arbitration bars and tested. Nitrogen was found to improve the physical properties and refine the grain-structure greatly. A typical set of values obtained from this series of tests follows:

Analysis Physical properties Or %N Doll. T.B.L. Brinell In the above table for cast steels, the following symbols are used: Defi. i'or deflection in inches. T. B. L. for transverse breaking load in thousands of pounds, and Brinell for the hardness values on the Brinell scale.

The presence of nitrogen in no way decreases the resistance of chromium steels to staining and certain types of corrosion, as I have determined by extensive tests of both wrought and cast steels.

In making all castings used in my tests, 1 have followed a new and special procedure which I have developed for applying my invention. A low carbon steel, according to my procedure, is melted in one of the usual types of furnaces. During the melting process the desired amount of nitrogen is introduced into the molten metal as chromium nitride or a high nitrogen ferrochrome alloy or other suitable nitride or mixture of nitrides. After the steel is melted itis held at a temperature sufiiciently above the melting point to allow the nitrogen liberated from the unstable nitrides to escape. By regulating the temperature of the molten steel, the content of the combined stable nitrides can be controlled to give solid cast metal, as the stability of the nitrides decreases with a rise in temperature. This method of producing high nitrogen steels avoids the formation of blowholes and excessive piping, since the nitrogen remaining in the steel is in an alloyed condition.

Although I have described a number of specific examples and disclosed the preferred form of my invention, it will be understood that these are merely representative embodiments, and do not tend to limit the scope of my real discovery and invention relative to a method generally applicable to the improvement of alloy steels containing a high content 01' chromium. I wish therefore to be limited only by the prior art and tile appended claims.

I claim:

1. An alloy steel comprising a preponderance of iron, about 15% to 35% chromium, about 0.10% to 2% carbon, and nitrogen in an amount not less than about 0.2% and not greater than about 0.65%, and the balance chiefly iron; said steel having throughout substantially the same concentration of nitrogen.

2. An alloy steel comprising a preponderance of iron, about 18% to 30% chromium, about 0.10% to 2% carbon, and nitrogen in an amount not less than about 0.2% and not greater than about 0.65%, and the balance chiefly iron; said steel having throughout substantially the same 'concentration of nitrogen.

..,3. An alloy steel comprising a preponderance of iron'rfrom about 18% to about 30% chromium. from about 0.10% to about 1% carbon, and from about 0.2% to about 0.4% nitrogen, the balance chiefly iron; said steel having throughout substantially the same concentration of nitrogen.

4. Alloy steel comprising from about 18% to about 30% of chromium, from about 0.10% to about 1% of carbon, from about 0.2% to about 0.4% of combined nitrogen, and the balance substantially iron; said steel having throughout substantially the same concentration of nitrogen.

5. Fine-grained high-chromium iron-base alloy casting resistant to staining, corrosion and high-temperature embrittling, and having approximately the composition: 15% to 35% of chromium, 0.10% to 2% of carbon, 0.20% to 0.40% of combined nitrogen, and the balance substantially iron; said casting having throughout substantially the same concentration oi nitrogen.

6. An alloy steel consisting of about 15% to 35% chromium, about 0.10% to 1% carbon, about RUSSELL FRANKS.